Generally, tuning circuits for receiving a video signal in a television receiver and the like require to have a relatively wide frequency bandwidth and thus use, for example, staggered tuning amplifiers. The staggered tuning amplifiers are a combination of plural stages of single tuning circuits, each adapted to choose a narrow frequency bandwidth signal, to provide a wide frequency bandwidth and a given amplification factor as a whole. Such amplifiers find wide applications not only in the above mentioned television receivers but also in intermediate frequency amplifiers for radar systems, etc.
A plurality of single tuning amplifiers constituting the above mentioned conventional staggered tuning amplifiers comprise a combination of resonance circuits, each including typically an inductor and a capacitor. By shifting the resonance frequencies by a given frequency from one another, a relatively flat, wide frequency bandwidth tuning characteristic may be obtained as a whole. Tuning frequencies in such staggered tuning amplifiers are determined by inductors and capacitors to be used as pairs. However, any attempt to obtain a wide frequency bandwidth by combining, for example, two stages of single tuning amplifiers with a certain frequency difference may result in a non-flat tuning characteristic with undesirable attenuation in the intermediate or center frequency range. This leads to a conclusion that the number of cascade connected, single tuning amplifier stages must be increased for wider frequency characteristic. In varying tuning frequencies of three or more stages of tuning amplifiers, the resonance frequency of each single tuning amplifier must be controlled in harmony with those in the other amplifiers, thereby making the control very complicated and subjecting to substantial change in characteristic when the tuning frequencies are varied. This is believed to be the reason why the tuning frequencies have never been varied in practical use. In a heterodyne system, for example, signal is processed after being converted into an intermediate frequency and thus the tuning frequency is maintained constant.